Influencing factors of the nonlinear sensing property of carbon humidity sensors

The humidity-sensing property of carbon black (CB)/hydroxyethyl cellulose (HEC) humidity sensors and its dependence on the chemical composition of the sample were investigated. Sorbitol was incorporated into the samples as a sensitizing agent and its effect on the nonlinearity of the devices‘ performance was studied. The measurement results of conductivity，sensitivity to humidity and complex impedance spectrum (CIS) suggest that the amounts of CB and sorbitol affect the sensing performance greatly. A nonlinear increase in resistance was observed at 80% humidity on the sample with 2% CB，and the sensitivity response shifts to lower humidity values when an appropriate amount of sorbitol is added. The sensor equivalent circuits derived from the CIS results can be modeled by a Warburg impedance in series with a number of resistance-capacitance circuits that correspond to bulk resistance，CB-grain-boundary resistance and contact resistance，respectively. The number of semicircles in the CIS increases with humidity level，which indicates different polarization mechanisms due to the adsorbed water.